The present invention relates to an electrophotographic image forming apparatus wherein a line-shaped image exposure means such as an LED, FL OR EL head and a developing means are located in the vicinity of an image forming body and thereby a toner image is formed on aforesaid image forming body.
An image forming apparatus such as a laser printer can operate by employing a writing system to conduct scanning by means of a laser beam as an image exposure means based on an image-preparation theory by means of an electrophotographic process.
Generally, there are two scanning methods by means of laser beams. One is a method in which laser beams emitted from a laser emitting element are changed to be a parallel beam through a collimator lens and aforesaid parallel beam is reflected by means of a polygonal mirror. Aforesaid scanning beam is caused to pass through an f-.theta. lens and a cylindrical lens to conduct image exposure on an image forming body which moves (secondary scanning) perpendicularly to the scanning direction so that a latent image is formed. The other method is one in which image exposure is conducted on an image forming body which is composed of an emission element such as an array-shaped LED and an image-forming element such as a Selfoc lens and which moves perpendicular with aforesaid array. Since the image exposure means employing the array-shaped emission element is compact due to no moving portion, it is frequently used in a color image forming apparatus in the vicinity of an image forming body in which plural image exposure means are provided and the array-shaped emission elements conduct image exposure corresponding to multiple colors such as Y, M, C and K.
In an array-shaped exposure optical system in which FL, EL or LED elements are arranged in a line, if an image is formed by means of multiple value modulation employing an LED array as an example in which the LED is arranged linearly, uneven streaking and uneven light amounts are caused due to arrangement error of each chip (an emission unit) inside the array. In addition, when plural LED arrays are provided in the vicinity of the image forming body so that multiple color toner images are superposed on the image forming body, alignment between each LED array is extremely difficult.